The invention relates to a method for controlling lowering of an implement of a working machine.
The invention is applicable on working machines within the fields of industrial construction machines, in particular wheel loaders and articulated haulers. Although the invention will be described with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other working machines having hydraulic working functions, such as dump trucks, excavators or other construction equipment.
A working machine is provided with a bucket, container or other type of implement for digging, lifting, carrying and/or transporting a load.
A wheel loader, for instance, has working functions driven by hydraulics, such as lifting and tilting of an implement arranged on a load arm unit. The load arm unit comprises a number of hydraulic cylinders for movement of the load arm and the implement attached to the load arm. A pair of hydraulic cylinders can be arranged for lifting the load arm and a further hydraulic cylinder can be arranged on the load arm for tilting the implement.
The wheel loader which usually is frame-steered has also a pair of hydraulic cylinders for turning/steering the wheel loader by pivoting a front part and a rear part of the wheel loader relative to each other.
In addition to the hydraulic cylinders, the hydraulic system of a wheel loader comprises one or more hydraulic machines (pumps) for providing hydraulic fluid to the hydraulic cylinders of the load arm unit and the steering unit.
By the use of a recovery unit in the hydraulic system, energy can be recovered by utilizing a return flow from one or more hydraulic cylinders. The recovery unit can be a hydraulic motor driven by the return flow. The hydraulic motor is then preferably connected to an electric generator. A disadvantage with prior art hydraulic systems having a recovery unit and already known methods for recovering energy in such a hydraulic system is however the fact that a relatively large recovery unit is needed to be able to handle the flow of hydraulic fluid. The flow of hydraulic fluid is proportional to the speed of the implement. For example, when the bucket of a wheel loader is lowered this operation can be associated with a relatively large flow of hydraulic fluid in comparison to other hydraulic functions in the system. This means that the recovery unit has to be “oversized” to be able to handle the return flow or the return flow (or at least a part thereof) has to be by-passed to tank without recovering any energy. In addition, the speed of the bucket has to be controlled without any unwanted instability in the system induced by the recovery function.
It is desirable to provide a method defined by way of introduction, by which method energy can be recovered during lowering of an implement when a relatively large hydraulic return flow is created at the same time as instability in the hydraulic system is counteracted.
By the provision of a method where the fluid communication between the piston side of the hydraulic cylinder and the piston rod side of the hydraulic cylinder is enabled, the hydraulic flow to the recovery unit can be reduced. Instead a “transformation” from flow to pressure takes place due to the fact that a part of the hydraulic fluid from the piston side can flow to the piston rod side of the hydraulic cylinder. In other words; the flow to the recovery unit will decrease at the same time as the pressure in the hydraulic cylinder will increase for a given external load on the hydraulic cylinder.
By the provision of a method using a hydraulic system where the piston side of the hydraulic cylinder and the control valve are connected to each other, the control valve and the recovery unit are connected to each other, and the piston rod side of the hydraulic cylinder is connected to the control valve and to the recovery unit in a point between the control valve and the recovery unit, fluid communication between the piston side of the hydraulic cylinder and the piston rod side of the hydraulic cylinder is enabled, at the same time as a desired counter pressure can be achieved by the recovery unit while having substantially same pressure at the piston rod side of the hydraulic cylinder and the inlet side of the recovery unit.
This will increase stability in the system, since in a hydraulic system it is preferred that the pressure is substantially the same in different parts of the system. Pressure zones with different pressures are to be avoided since the control components of the hydraulic system are associated with some time-delay which can bring the components out of phase and induce instability to the system.
In a preferred embodiment of an aspect of the invention, the method comprises the step of controlling a pressure at the piston rod side of the hydraulic cylinder resulting in a minimal pressure drop over the control valve required to obtain the requested lowering speed, and controlling the recovery unit to provide a counter pressure resulting in the calculated minimal pressure drop pressure at the piston rod side of the hydraulic cylinder, and preferably this is achieved by using a control valve which valve is able to give the desired flow substantially independently of the pressure drop over the valve, at least for a certain pressure drop interval. In other words; the control valve can preferably be adjustable to give the desired flow for different pressure drops over the valve, and thereby the desired speed of the implement can be achieved for different pressure drops over the control valve. The control valve is preferably some kind of pressure compensated valve.
By controlling the pressure at the piston rod side by means of the recovery unit in a way resulting in a minimal pressure drop over the control valve required to obtain the requested lowering speed, the energy losses can be minimized at the same time as the desired speed can be achieved.
The invention also relates, according to an aspect thereof, to a computer program and a computer readable medium for performing the steps of the method according to the invention.
Further advantages and advantageous features of the invention are disclosed in the following description.